The solid lines in Fig. 6(c) are the curves obtained by fitting. Obviously, the fitting results are well consistent with the experiment data. Furthermore, it can be observed that the charge carrier density of the two OSCs decreases with the increase in delay time, which is due to the charge carrier recombination. The parameters
n0,
t, and
a for the OSCs can be obtained through fitting and they are listed in Table 2. It can be noted that in contrast to the OSCs without BTA2, the introduction of BTA2 into the OSCs resulted in the
n0 of the ternary OSCs increasing from 8.22×10
16 to 9.34×10
16 cm
‒3. However, as the delay time lengthened, the charge carrier density in the ternary OSCs declined faster than that in the binary OSCs. This implies that after the free charge carrier generation, the introduction of the third component will result in more charge carriers loss in the processes of charge carrier transport and extraction in the ternary OSCs. The dispersion parameter
a of the binary and ternary OSCs was 0.897 and 0.821, respectively, indicating that the recombination belongs to trap-limited bimolecular recombination with regard to the two OSCs (when
a is equal to 1, the recombination mechanism is trap-free bimolecular recombination and when
a is less than 1, the recombination mechanism is trap-limited bimolecular recombination [
39]). The charge carrier bimolecular recombination rate
gbi can be calculated according to formula (3), as shown in Fig. 6(d). We can observe that the plots for the binary and ternary OSCs demonstrate a marginal decrease with the increase in delay time. However, the
gbi of the binary OSCs (5.94×10
‒13 cm
3·s
‒1) is significantly lower than that of the ternary OSCs (9.18×10
‒13 cm
3·s
‒1), resulting in the recombination lifetime
t increasing from 7.79
ms for the ternary OSCs to 9.14
ms for the binary OSCs (Table 2). The results imply that in contrast to the binary OSCs, the charge carriers corresponding to the ternary OSCs are more likely to be trapped and bimolecular recombination occurs after the charge carrier generation. Although the initial charge carrier density of the ternary OSCs excited by laser is higher, the carrier lifetime is shorter and decay is faster in the process of transport and extraction. Therefore, the introduction of 10% mass concentration of the third component, BTA2, into the PTB7:PC
71BM solar cell is disadvantageous to the transport and extraction after the generation of carriers.